In what is being called the largest individual gift to support same-sex marriage in the U.S., Jeff and MacKenzie Bezos have donated $2.5 million to a Washington state coalition working to approve Referendum 74. That measure, on the November ballot, is designed to uphold the state law protecting the rights of same-sex couples to marry.

The donation doubles the amount of money raised by Washington United for Marriage.

“I am thrilled by Jeff and MacKenzie’s staggeringly generous donation,” said Jennifer Cast, WUM’s volunteer finance co-chair and a former Amazon.com manager. “Their support of our efforts to approve R74 comes at an important time and will have great impact. I am deeply grateful to both of them as this donation is going to help us make history.”

A separate organization by the name of Preserve Marriage Washington is looking to reverse the state’s law on same-sex marriage, which was passed earlier this year and signed by Gov. Chris Gregoire. Preserve Marriage writes on their Web site:

Marriage is a special relationship reserved exclusively for heterosexual unions because only the intimate relationship between men and women has the ability to produce children as a result of that sexual union. The marriage relationship is inherently different than a same-sex relationship in this important regard.

The New York Times reports that Cast, a 50-year-old lesbian mother of four children, sent an email to Bezos on Sunday night pleading with the billionaire to get involved in the effort.

“I want to have the right to marry the love of my life and to let my children and grandchildren know their family is honored like a ‘real’ family,” Cast wrote in the email. “We need help from straight people. To be very frank, we need help from wealthy straight people who care about us and who want to help us win.”

Bezos, who has been described by those who know him as a libertarian, has been criticized in the past for not giving back to the community. He did donate money in 2010 to defeat an income tax measure in Washington that would have taxed some of the wealthiest residents in the state.

But it was nowhere near the size of the $2.5 million donation to Washington United for Marriage. Bezos also has endowed scholarships recently at the University of Washington, and last summer gave $10 million to Seattle’s Museum of History and Industry.

Last December, Bezos donated $15 million to create the Bezos Center for Neural Circuit Dynamics at Princeton University.

Asked about the donation to the Washington United for Marriage effort, Amazon.com spokesperson Drew Herdener said that it’s a personal decision by Jeff and MacKenzie Bezos and that the couple “feel strongly about the issue.”

Microsoft’sSteve Ballmer and Bill Gates also have each made $100,000 donations to the effort to preserve same-sex marriage in Washington state.

UPDATE: State Sen. Ed Murray, who led legislative efforts to pass the Marriage Equality Act earlier this year, offered this statement on the donation by the Bezos family.

“Strategically it couldn’t come at a more important time. It assures the campaign will have the basic resources to defend marriage equality. It will also make it so much easier for other business leaders to do the right thing and join the campaign.”